Posterior spinal arthroplasty system

ABSTRACT

A lumbar disc prosthesis is provided including a first member with a vertebral disc contact surface and a recessed portion on an opposing surface thereof; a second member with a vertebral disc contact surface and a protruding portion on an opposing surface thereof. The protruding portion of the second member engages with the recessed portion of the first member in use. A facet joint prosthesis is provided, including a first member for attachment to a first posterior lumbar disc in use and a second member for attachment to a second posterior lumbar disc in use, where at least a part of the first member is telescopically mounted in at least a part of the second member in use.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.11/397,756, filed Apr. 4, 2006, which is a continuation-in-part of U.S.application Ser. No. 11/203,259, filed on Aug. 12, 2005 now abandoned,which is a continuation-in-part of U.S. application Ser. No. 10/970,091,filed on Oct. 21, 2004 now abandoned, the entirety of all of which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

n/a

FIELD OF THE INVENTION

This invention relates to devices and surgical methods for the treatmentof various types of spine pathologies. It deals with the development ofan artificial facet joint, and an artificial lumbar disc replacementthat is specifically designed to be inserted from a posterior approachto the spine. It also deals with the development of an artificial discreplacement that can be inserted from an anterior approach to the spine.

BACKGROUND OF THE INVENTION

Back pain affects 40% of the population. Up to 20% of the populationvisit their family doctor requesting help with their back problem. Up to30% of patients continue to complain of significant back pain at oneyear following the onset of their symptoms.

Although the majority of patients have minor sprains or strains whichare self limiting, a significant number of patients go on to developsevere chronic mechanical lower back pain which is caused byinflammatory changes in the lumbar disc associated with degeneration.

Another group of patients with degenerative spine disease go on todevelop degenerative spondylolisthesis and spinal stenosis. This is anarrowing of the spinal canal caused primarily by degenerative changesin the facet joint, combined with a loss of normal disc height andbuckling of the ligamentum flavum.

Degeneration occurs in a spinal segment. The spinal segment consists ofthe lumbar disc anteriorly and two facet joints posteriorly. This istherefore called a three joint complex. Degenerative changes in the disccan lead to changes in the facet joint and vice versa. In patients withsignificant lumbar disc degeneration, the facet joints are also usuallydegenerate.

Pain occurs from all components of the three joint complex, includingthe facet joints and the disc. The facet joint is in fact a synovialjoint and suffers from the problems that are known to affect othersynovial joints in the body like the hip and the knee. The facet jointparticularly contributes to degenerate spondylolisthesis and commonlyoccurs at levels where the facet joints are sagittally orientated, forexample at the L4/5 level.

After failing all the conservative treatments available, a minority ofpatients with back pain or leg pain will go on to require surgicalintervention. For patients with predominantly lower back pain who have adegenerative lumbar disc, some surgeon's consider the solution lies inremoving the pain generator which is the disc and restoring normalloading across the disc by doing an inter-body stabilisation procedure.

The two types of inter-body stabilisation procedure currently availableare an artificial disc replacement performed anteriorly and inter-bodyfusions performed anteriorly and/or posteriorly. These inter-bodystabilisation procedures are often combined with decompression of thespinal canal and the nerve roots if there is nerve root impingement.

As far as inter-body fusions are concerned, there are two basicstrategies that surgeons adopt. The first is to perform an anteriorinter-body fusion combined with posterior stabilisation externally ofthe spinal canal. Anterior inter-body fusion on its own is stillquestioned because it does not provide a posterior tension band. Analternative strategy is a posterior lumbar inter-body fusion, where theentire inter-body fusion procedure is performed from behind and it iscombined with neural decompression as well as removal of thedegenerative facet joints. Posterior lumbar inter-body fusion alsoprovides a posterior tension band. This strategy therefore deals withall three joint components which can generate possible pain at the disclevel, including the lumbar disc, the neural structures and the facetjoints.

When it comes to lumbar disc arthroplasty procedures, these areperformed via an anterior lumbar approach. The disc is removed and anartificial lumbar disc is inserted into the space. This removes the paingenerator and allows normal loading across the disc, as well as allowingsome movement at this level. The advantage of this is to reduce thestrains on the disc above and therefore reduce the chances of adjacentsegment degeneration. Several studies have shown that adjacent segmentdegeneration can occur above a fused segment because of the increasedloads being transmitted to this level.

One of the disadvantages of anterior lumbar arthroplasty is that thefacet joints at this level continue to move and also continue to act asa pain generator. In addition, if there is any neural impingement, thesesymptoms can continue. The other disadvantage of anterior lumbararthroplasty is that the majority of spine surgeons are not familiarwith the anterior approach, and although complications are uncommon,they can be life and limb threatening when they do occur.

There is therefore a concern among researchers and the surgicalcommunity, that long term results of anterior lumbar disc arthroplastymay be compromised by progressive degeneration of the facet joint at thesame level. In addition, after lumbar disc arthroplasty, severalpatients continue to complain of facet joint pain because of increasedloads being placed on the facet joint as a result of the surgicalprocedure.

At present there is no posterior lumbar arthroplasty procedureavailable. It is therefore an aim of the present invention to provide anartificial lumbar disc that can be inserted posteriorly, therebydelivering the advantages of approaching the spine posteriorly andremoving the disadvantages associated with approaching the spineanteriorly.

It is a further aim of the present invention to provide a facet jointreplacement prosthesis.

It is a yet further aim of the present invention to provide a lumbarprosthetic system that deals with the painful disc, the neuralimpingement and the painful facet joints by providing a combination of alumbar disc prosthesis and a facet joint prosthesis as a single unit.

It is a further aim of the present invention to provide an artificialdisc prosthesis that utilises one or more of the features and advantagesprovided by the posteriorly inserted disc but which can be insertedanteriorly.

It is a yet further aim of the present invention to provide a prostheticsystem that deals with the painful disc, the neural impingement and thepainful facet joints by providing a combination of a disc prosthesis anda facet joint prosthesis as a single system that works together.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided adisc prosthesis, said disc prosthesis including a pair of disc members,the first member of said disc pair having a vertebral endplate contactsurface and a recessed portion on an opposing surface thereof, thesecond member of said disc pair having a vertebral endplate contactsurface and a protruding portion on an opposing surface thereof, theprotruding portion of the second member engaging with the recessedportion of the first member in use, and wherein the inner or opposingsurface of at least the first disc member is provided with at leastthree sections; a middle section and at least two end sections, therecessed portion being provided in the middle section and the thicknessor depth of the middle section being less compared to the two endsections thereof.

In one embodiment the end sections, typically located adjacent theanterior and posterior edges of the prosthesis are substantiallysurfaces which are planar in form. Thus, the planar ends sections aredifferent in form to the curved middle recessed section.

The geometry of the middle and end sections of at least the first discmember allows contact to be made between the recessed and protrusionportions of the first and second disc members and for a gap to be formedadjacent the end sections of the disc members when the prosthesis is ina neutral position. When the prosthesis is in extension (i.e. the upperor first disc member moves posteriorly relative to the lower or seconddisc member), the gap between the end section at the anterior endincreases, thereby causing the annulus anteriorly to tighten. Due to thegeometry of the planar slopes within the prosthesis, the annulus willtighten not only anteriorly in flexion but laterally as well. Thisresults in a physiological stop to further extension. Similarly inflexion the upper disc member moves anteriorly relative to the lowerdisc member, resulting in an increased gap posteriorly between the twodisc members, and this causes the annulus posteriorly and laterally totighten, resulting in a physiological stop to further flexion.

In one embodiment the middle section is located substantially centrallyof the prosthesis. The end sections can be of substantially the samedimensions or substantially different dimensions.

Preferably the inner or opposing surface of the second disc member isprovided with at least three sections; a middle section and at least twoend sections, the protruding portion being provided in the middlesection of the disc member.

In one embodiment the boundaries of the middle and end sections of oneor both disc members can be arranged transversally thereof (i.e. inmedial to lateral plane).

In one embodiment the boundaries of the middle and end sections of oneor both disc members are arranged in anterior/posterior plane.

The middle and end sections of one or both disc members can be arrangedalong substantially the entire length thereof, or the middle and endsections of one or both disc members can be arranged in a substantiallycentral part of the disc members.

In the embodiment where the prosthesis is inserted anteriorly, themiddle or recessed portion of the first member is substantially curvedand the curvature of the medial part of said recessed portion issubstantially symmetrical to the curvature of the lateral part of saidrecessed portion. Thus, the lateral part of the recessed or middleportion has curvature corresponding to an arc of a circle which has aradius substantially equal to that of an arc of a circle correspondingto the curvature of the medial part of the recessed portion or middlesection.

Preferably an anterior part of the recessed portion or middle section issubstantially symmetrical to a posterior part of the recessed portion.The symmetry of the parts of typically about the mid point of the middlesection.

In the anteriorly inserted embodiment, the protrusion portion of thesecond member is typically of substantially the same shape and/ordimensions as the recessed portion of the first member.

In the anteriorly inserted embodiment, the protrusion portion ispreferably a rugby ball type shape with the radius of the curve of theprotrusion portion in the medial-lateral plane being larger than theradius of the curve of the protrusion portion in the anterior-posteriorplane. Thus, the protrusion portion is typically asymmetrical in theanterior-posterior plane compared to the medial to lateral plane.

Preferably one or both end sections of the inner or opposing surfaces ofsaid first and/or second members are provided at an acute angle to thehorizontal. For example, the inner end section surface of said firstmember can slope downwardly towards one or both ends of said member. Theinner end section surface of said second member can slope downwardlytowards the one or both ends of said member.

In the anteriorly inserted embodiment, the disc members are formed so asto have lordosis provided therein, such that the overall posteriorheight of the disc pair when assembled together is less than theanterior height.

Further preferably the outer and/or inner surfaces of said second discmember form a narrowing taper towards one or both ends of said discmember.

Preferably one or both ends of the first and/or second disc members havea narrowing taper (i.e. the ends of the prosthesis as a whole has anarrowing taper). This narrowing taper is as a result of the outer orvertebral end plate contact surface of said first and/or second discmember being provided at an acute angle with respect to the horizontalat one or both ends. These angled surfaces are preferably substantiallyplanar in form. For example, the outer or vertebral end plate contactsurface of the first disc member slopes downwardly towards the ends ofthe disc member and/or the outer or vertebral end plate contact surfaceof the second disc member slopes upwardly towards the ends of the discmember. The narrowing taper of the prosthesis is particularlyadvantageous as it allows easy insertion of the prosthesis in the discspace via a posterior route.

In one embodiment, such as for example in the posteriorly insertedprosthesis, the narrowing taper is provided at an anterior end of theprosthesis. Thus, the outer surfaces or vertebral endplate contactsurfaces of the disc member(s) slope towards the opposing surfacesthereof at the anterior end of the disc member(s). This provides a “leadin” feature which increases the ease with which the front of theprosthesis can be inserted via a posterior route into the disc space. Inthis embodiment, where the disc is inserted through the posterior route,the overall height of the anterior part of the disc pair is greater thanthe posterior height, so as to place the two vertebral endplates inlordosis, when the discs are inserted.

In one embodiment, a narrowing taper is provided at the posterior end ofthe prosthesis. Thus, for example, the outer surfaces or vertebralendplate contact surfaces of the disc member(s) slope towards theopposing surfaces thereof at the posterior end of the disc member(s).The posterior angled surface allows the prosthesis in the neutralposition to be placed such that the vertebral end plates are inlordosis.

The posterior end of the prosthesis typically slopes or tapers in anopposite direction to the anterior end.

Preferably the anterior end slope is substantially smaller than theposterior end slope in the embodiment where the disc prosthesis isinserted posteriorly.

Further preferably the outer and inner surface of said first disc memberform a narrowing taper adjacent an end of said disc member in whichattachment screws are to be located there through. The screw insertionhole is located posteriorly in the embodiment where the discs areinserted from the posterior route, and anteriorly where the discs areinserted through the anterior route.

Preferably the vertebral endplate contact surface of the first and/orsecond members of each disc pair is provided with attachment means forallowing attachment of the disc member to an adjacent disc in use.

The attachment means can include any or any combination of one or morescrews, one or more apertures for the location of screws and/or anyother suitable attachment device therewith, one or more tapered members,spikes or fins and/or any other conventional attachment apparatus.

In one embodiment the vertebral endplate contact surface of the firstmember of each disc pair includes a curved or convex shaped portionthereon. Preferably the disc pair is a lumber disc prosthesis which canbe inserted anteriorly but preferably posteriorly.

Preferably the contours of the curved or convex shape portioncorresponds substantially (i.e. is substantially complementary) to theconcave or curved shape portion of the end surface of the vertebral bodywhich the vertebral endplate contact surface of the prosthesis engageswith in use. This provides an improved fit of the prosthesis in the discspace.

Preferably the longitudinal axis of the curved or convex shaped portionis parallel to the anterior-posterior axis of the vertebral endplatecontact surface.

Preferably the height of the curved or convex shaped portion is greatertowards the medial side than the lateral side. Thus the vertebralendplate contact surface slopes upwardly from the lateral side to themedial side.

The angle between the vertebral endplate contact surface of the firstand second members or upper and lower disc members is approximately 7degrees on the lateral side, but can vary.

Preferably the curved or convex shape portion on the vertebral endplatecontact surface is substantially symmetrical about theanterior-posterior axis and/or medial to lateral axis.

The first and second members of the disc pair typically correspond toupper and lower disc members. The superior surface of the lower discmember has a protrusion thereon and the inferior surface of the upperdisc member has a captive recess or socket thereon.

Preferably the recessed portion has two sloped surfaces associatedtherewith, typically corresponding to the end sections thereof, onesurface leading anteriorly to the edge of the inferior surface and onesurface leading posteriorly to the edge of the inferior surface. Thesesloping surfaces can be planar or can be slightly concave in form, butgenerally do not form the arc of a radius.

Preferably the disc prosthesis can be used in the cervical and/or lumbarspine in the embodiment where the prosthesis is inserted anteriorly.

Preferably the disc prosthesis can be used in the lumbar and/or thoracicspine where the prosthesis is inserted posteriorly.

The disc prosthesis of the present invention can be used to replacelumbar discs either anteriorly or posteriorly, and cervical discsanteriorly.

In the embodiment where the disc prosthesis of the present invention isinserted posteriorly, two disc pairs are required.

Thus, according to a second aspect of the present invention there isprovided a lumbar disc prosthesis, said lumbar disc prosthesis includinga pair of disc members, the first member of said disc pair having avertebral endplate contact surface and a recessed portion on an opposingsurface thereof, the second member of said disc pair having a vertebralendplate contact surface and a protruding portion on an opposing surfacethereof, the protruding portion of the second member engaging with therecessed portion of the first member in use, and wherein the spinal discprosthesis includes a further pair of disc members, said further pair ofdisc members also including first and second members.

Preferably the disc pairs are minor images of each other.

Thus, the second aspect of the present invention provides a lumbar discprosthesis having two disc member pairs and thus two separatearticulating portions, one articulating portion on each pair of discmembers. The disc member pairs are each independently inserted into thedisc space on either side of the dural sac in use through thetrans-foraminal or posterior route bilaterally and are provided a spaceddistance apart in use to allow accommodation in accordance with theanatomy of the lateral aspect of the disc space. Thus, the discprosthesis can be inserted through the posterior route whilst takinginto account the neural anatomy posteriorly.

The lumbar disc prosthesis of the present invention can be inserted atall levels between L2 and the sacrum, typically depending on the levelof expertise of the operating surgeon.

Preferably the first members of each disc pair are provided in the leftand right areas of the disc space respectively in use. The secondmembers of each disc pair are also provided in the left and right areasof the disc space respectively in use. Thus, each pair of membersconstitutes a left and right lumbar disc prosthesis.

In the posteriorly inserted lumbar disc prosthesis, the recessed portionor middle section of the first member is substantially curved and thecurvature of the medial part of said recessed portion is asymmetrical tothe curvature of the lateral part of said recessed portion.

The purpose of this asymmetry in the medial-lateral plane is to allow“capture” of the protruding portion of the second member, such that whentwo pairs of disc prostheses are placed in the patient, the twovertebrae can move from side to side as well as anteriorly andposteriorly.

In the posteriorly inserted embodiment, preferably the lateral part ofthe recess has curvature corresponding to an arc of a circle which has aradius greater that of an arc of a circle corresponding to the curvatureof the medial part of the recessed portion. Further preferably theradius of the lateral part is at least twice as big as the radius of themedial part.

Preferably an anterior part of the recessed portion is substantiallysymmetrical to a posterior part of the recessed portion.

In the posteriorly inserted embodiment, the protrusion portion of thesecond member is typically of different shape and/or dimensions to therecessed portion of the first member. Thus, in one embodiment theprotrusion portion is asymmetrical or forms an asymmetrical fit with therecessed portion. In an alternative embodiment the protrusion portion issymmetrical or forms a symmetrical fit with the recessed portion.

In the posteriorly inserted embodiment, the protrusion portion ispreferably substantially dome shaped and makes contact with only a partof the recessed portion when assembled. However, the protrusion portioncould contact a substantial part of the recessed portion when assembledif required.

Preferably the recessed portion is substantially an inversed dome shape.

In the posteriorly inserted embodiment, the protrusion portion istypically symmetrical in the anterior-posterior plane and in the medialto lateral plane. The curvature in the medial to lateral plane can bethe same or different to that in the anterior to posterior plane.

According to a further aspect of the present invention there is provideda lumbar disc prosthesis, said disc prosthesis including a pair of discmembers, the first member of said disc pair having a vertebral endplatecontact surface and a recessed portion on an opposing surface thereof,the second member of said disc pair having a vertebral endplate contactsurface and a protruding portion on an opposing surface thereof, theprotruding portion of the second member engaging with the recessedportion of the first member in use, and wherein the vertebral endplatecontact surface of said first member includes a convex shaped portionthereon.

The lumbar disc prosthesis can be used in conjunction with a facet jointprosthesis also provided posteriorly to provide a system which can worktogether as a single unit to replace the painful disc, overcomes neuralimpingement and painful facet joints.

The facet joint prosthesis is formed such that the arc of rotation ofthe facet joint substantially matches the arc of rotation of theposterior disc prosthesis. As such, the “Instant Axis of Rotation” (IAR)of the two prosthesis forming the unit is substantially the same.Furthermore, the facet joint prosthesis stabilises and can unload theposterior disc prosthesis by pre-loading the segment into kyphosis.

Preferably the facet joint prosthesis typically includes a first memberfor attachment to a first vertebra of a corresponding disc and a secondmember for attachment to a second vertebra of a corresponding disc inuse, and wherein at least a part of said first member is telescopicallyor slidably mounted in at least a part of said second member in use.

Preferably the first and second members are elongate members and theprovision of one telescopically mounted in the other allows the distancebetween parts of the first and second members to be increased and/ordecreased as required.

The first vertebra is typically an upper vertebra and the secondvertebra is typically a lower vertebra.

The facet joint prosthesis allows replacement of existing facet jointsto be undertaken at all lumbar levels from T12 to the sacrum.

The interconnecting first and second members are formed such that theycan articulate to allow flexion-extension, small degrees of rotation andside to side flexion.

Preferably securing means are provided for insertion of the first andsecond members into each of the vertebral bodies above and below thedisc between which the facet joint prosthesis is to be located. Thefirst and second members can then be secured to said securing means. Aplurality of first and second members can be attached to the securingmeans if required to form a stack, thereby allowing facet jointreplacement at multiple levels within the spine.

The securing means can include any suitable type of surgical securingdevice, such as a pedicle screw.

In one embodiment the ends of the first and/or second members which areto be attached to the vertebral bodies are provided with at least oneaperture through which the securing means are located.

In one embodiment the ends of the first and/or second members which areto be attached to the vertebral bodies are provided with substantiallyrounded and/or continuous ends to allow a clamp and pedicle screw orother securing means to be associated with the same.

Preferably locking means are provided on the first and/or second membersto allow locking of the members together. The locking means can includeany or any combination of one or more interengaging portions, one ormore locking screws and/or the like.

In one embodiment the locking means includes one or more clamping rings.The clamping rings are located around the first and second members andcan clamp the members in a suitable position relative to each other byapplying a suitable clamping force thereto. The clamping force can beprovided using one or more screws and/or the like.

Preferably each clamping ring is of sufficient size and dimensions tosubstantially encompass the outermost member. The free ends of theclamping ring can be moved towards or apart from each other using saidone or more screws and/or other suitable engagement means.

Preferably two clamping rings are used to clamp the first and secondmembers together. Movement of the clamping rings relative to each otheron the members determines the range of movement of the first memberrelative to the second member and thus the range of movement of thefacet joint prosthesis. Use of the clamping rings allows the prosthesisto achieve micro-motion or full motion. It also allows the prosthesis toact as a fusion rod or a dynamic rod, thereby allowing the prosthesis tobe multi-functional and more flexible in its application.

Preferably guide means are provided on the first and/or second membersto allow relative movement therebetween to be controlled or guided. Theguide means can include a slot or enclosed channel provided on one ofsaid members in which a rod or protrusion provided on the other of saidmembers is slidably mounted. This is to prevent the two members of thefacet prosthesis from disassociating from each other.

The clamping rings can be used to limit movement of the protrusion inthe slot or enclosed channel or male member.

Preferably an interior surface of the clamping ring is provided with aprotrusion part thereon which locates in said channel or slot whenfitted to the device.

In one embodiment the slot provided on one of said members can be openended.

Preferably a stop member is located in the channel of said secondmember. The stop member can act as a shock absorber. The stop member ispreferably removably located in said channel.

In one embodiment a sleeve, made of any suitable material, such as forexample silicon, plastic and/or the like can be present to cover thefacet articulation.

According to a further aspect of the present invention there is provideda lumbar disc prosthesis, said lumbar disc prosthesis including a pairof disc members, the first member of said disc pair having a vertebralendplate contact surface and a recessed portion on an opposing surfacethereof, the second member of said disc pair having a vertebral endplatecontact surface and a protruding portion on an opposing surface thereof,the protruding portion of the second member engaging with the recessedportion of the first member in use, and wherein the recessed portion ofthe first member is substantially curved and the curvature of the medialpart of said recessed portion is asymmetrical to the curvature of thelateral part of said recessed portion.

According to a yet further aspect of the present invention there isprovided a facet joint prosthesis, said prosthesis including a firstmember for attachment to a first disc in use and a second member forattachment to a second disc in use, and wherein at least a part of saidfirst member is telescopically or slidably mounted in at least a part ofsaid second member in use.

The facet joint prosthesis can be used alone or in combination with thedisc prosthesis of the present invention. The facet joint replacementprocedure of the present invention alone will have a role in thetreatment of patients with spinal stenosis and adjacent level discdisease, where some stability is required at the disc level withoutfusing a particular disc segment. The facet prosthesis can partiallyconstraining certain degrees of motion.

According to further independent aspects of the present invention thereis provided a prosthesis system including a disc prosthesis and a facetjoint prosthesis as hereinbefore described; a first or upper discmember; a second or lower disc member; a method of insertion of a lumbardisc prosthesis in a patient via a posterior route; a method ofinsertion of a facet joint replacement prosthesis in a patient via aposterior route and/or anterior route; and a method of insertion of alumbar disc prosthesis in a patient via an anterior route;

According to an aspect of the present invention there is provided aprosthesis system, said system including a disc prosthesis and facetjoint prosthesis, said disc prosthesis including a pair of disc members,the first member of said disc pair having a vertebral end plate contactsurface and a recessed portion on an opposing surface thereof, thesecond member of said disc pair having a vertebral endplate contactsurface and a protruding portion on an opposing surface thereof, theprotruding portion of the second member engaging with the recessedportion of the first member in use, said facet joint prosthesisincluding a first member for attachment to a first posterior lumbar discin use and a second member is provided for attachment to a secondposterior lumbar disc in use, and wherein at least a part of said firstmember is telescopically mounted in at least a part of said secondmember in use.

It will be appreciated by persons skilled in the art that the upper orlower disc member in any of the embodiments can be used alone ifrequired.

Any of the abovementioned features can be used alone or in combinationwith each other in a prosthesis according to the present invention.

Thus, the present invention overcomes the problems and disadvantagesassociated with current disc replacement strategies. It has all thebenefits associated with posterior lumbar inter-body fusion surgery, butat the same time it allows movement at that level and reduces the strainon adjacent discs and the risks of adjacent segment disc failure. Thisinvention also addresses all three pain generators at the lumbar disclevel including the degenerative disc, the impingement of the neuralstructures, and the facet joint.

BRIEF DESCRIPTION OF THE DRAWINGS

Generally, FIGS. 1-30 and 59 a-59 e relate to a lumbar disc prosthesisaccording to embodiments of the present invention for insertion via aposterior route, FIGS. 31-52 b and 60 a-61 relate to a facet jointreplacement prosthesis according to embodiments of the presentinvention; and FIGS. 53 a-58 e show lumbar and cervical disc prosthesisaccording to embodiments of the present invention for insertion via ananterior route. More particularly,

FIG. 1 is a side view of a lumbar disc prosthesis according to oneembodiment of the present invention;

FIGS. 2-4 show the prosthesis in FIG. 1 with the upper disc member inextension, flexion and in a neutral position respectively;

FIGS. 5 and 6 show the superior surfaces of the upper members of theleft and right lumbar prosthesis pairs respectively;

FIGS. 7 and 8 show rear views of the left and right lumbar prosthesispairs respectively;

FIGS. 9 and 10 show front views of the left and right lumbar prosthesispairs respectively;

FIGS. 11 and 12 show oblique views of the left and right lumbarprosthesis pairs respectively;

FIGS. 13 and 14 show the superior surfaces of the lower members of theleft and right lumbar prosthesis pairs respectively;

FIGS. 15 and 16 show rear views of the lower members of the left andright lumbar prosthesis pairs respectively;

FIG. 17 is a side view of the lower member of a lumbar prosthesis;

FIGS. 18 and 19 show front views of the lower members of the left andright lumbar prosthesis pairs respectively;

FIGS. 20 and 21 show oblique views of the lower members of the left andright lumbar prosthesis pairs respectively;

FIG. 22 is a side view of an upper member of a lumbar prosthesis pair toillustrate the angulations and geometry of the inferior surface thereof;

FIG. 23 is a front view of the upper member of the lumbar prosthesispair to illustrate the radius in the medial and lateral part of thecurvature of the inferior surface;

FIG. 24 is an oblique view of the upper member of the lumbar prosthesispair to further illustrate the shape of the inferior articulatingsurface thereof;

FIG. 25 shows a perspective view of the inferior surface of the uppermembers of the left and right lumbar prosthesis pairs respectively;

FIGS. 26 a-d illustrates a) a solid inverse representation of the recessportion shape on the inferior surface of the upper member of theprosthesis b) an axial view of the prosthesis c) a side view and d) afront view showing the medial/lateral asymmetry of the prosthesis;

FIG. 27 is a cross section through the upper member of a lumbarprosthesis pair showing the medial lateral curvature of the articulatingsurface;

FIG. 28 show front views of the left and right lumbar prosthesis pairsrespectively in an alternative embodiment with the screws replaced byfin members;

FIGS. 29 a and 29 b illustrate oblique views of the left and rightlumbar prosthesis pairs in FIG. 28 joined together and spaced apartrespectively;

FIGS. 30 a-30 f show a) superior surfaces of the upper members of theleft and right lumbar prosthesis pairs in FIG. 28 b) a side view c) afront view d) a further side view e) inferior surfaces of the lowermembers of the left and right lumbar prosthesis pairs and f) obliqueviews of the pairs;

FIG. 31 is a side view of a facet joint replacement prosthesis accordingto an embodiment of the present invention;

FIGS. 32-36 show a front view, end view, side view, rear view andoblique view respectively of the second or female member of the facetreplacement prosthesis;

FIG. 37 is a cross sectional view of the second or female member of thefacet replacement prosthesis taken along the line marked A-A in FIG. 34;

FIGS. 38-42 show a front view, end view, side view, rear view andoblique view respectively of the first or male member of the facetreplacement prosthesis;

FIG. 43 is a cross sectional view of the first or male member of thefacet replacement prosthesis taken along the line marked A-A in FIG. 40;

FIGS. 44 a-44 d illustrate a side view, front view, perspective viewfrom the rear and perspective view from the front of the facet jointreplacement prosthesis in FIG. 31;

FIGS. 45 a and 45 b illustrate the ability of the male member to movewithin the female member and produce a flexion and extension movement ofthe vertebral body respectively;

FIG. 46 illustrates end views of the facet replacement prosthesisshowing the ability of the male and female components to undergo limitedrelative rotational movement;

FIG. 47 illustrates front views of the facet replacement prosthesisshowing the ability of the male and/or female components to undergo sideto side flexion;

FIGS. 48 a-48 d show a side view, front view, perspective view from therear and perspective view from the front of facet joint prostheses whenstacked respectively;

FIG. 49 is an enlarged view of the ends of the female and male membersof the facet joint prosthesis;

FIG. 50 is a cross sectional view taken through a pedicle screw attachedto a female member of a facet joint prosthesis;

FIG. 51 is a perspective view of a facet joint replacement prosthesisaccording to a further embodiment of the present invention;

FIGS. 52 a and 52 b show perspective views of the facet joint prosthesisin FIG. 51 in a retracted and extended position respectively;

FIGS. 53 a-53 f show a lumbar disc prosthesis for insertion via ananterior route, particularly a top view of an upper disc member,anterior view of a pair of disc members, side view of a pair of discmembers, posterior view of a pair of disc members, base view of a lowerdisc member, and anterior perspective view of a pair of disc membersrespectively;

FIGS. 54 a-54 f show the lower disc member in FIGS. 53 a-53 f,particularly a top view, anterior view, side view, posterior view, baseview and perspective view respectively;

FIGS. 55 a-55 f show the upper disc member in FIGS. 53 a-53 f,particularly a top view, anterior view, side view, posterior view, baseview and perspective view respectively;

FIGS. 56 a-56 e show a cervical disc prosthesis for insertion via ananterior route, particularly a top view of an upper disc member,anterior view of a pair of disc members, side view of a pair of discmembers, posterior view of a pair of disc members, base view of a lowerdisc member, and anterior perspective view of a pair of disc membersrespectively;

FIGS. 57 a-57 e show the lower disc member in FIGS. 56 a-56 e,particularly a top view, anterior view, side view, posterior view, baseview and perspective view respectively;

FIGS. 58 a-58 e show the upper disc member in FIGS. 56 a-56 e,particularly a top view, anterior view, side view, posterior view, baseview and perspective view respectively;

FIGS. 59 a-59 e illustrates a further embodiment of first and secondmember disc pairs with the upper vertebral endplate contact surfacehaving a convex shaped portion provided thereon;

FIGS. 60 a-60 c illustrate a perspective view and partial crosssectional view with clamping rings separated and a partial crosssectional view with the clamping rings together respectively; and

FIG. 61 illustrates clamping rings according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Lumbar Disc Prosthesis for Insertion Via a Posterior Route

Referring firstly to FIGS. 1-30, there is illustrated a lumbar discprosthesis 2 which can be inserted into a lumbar disc space via aposterior route as a replacement for a diseased and/or damaged lumbardisc.

The disc prosthesis 2 includes two pairs of disc members, each pairincluding an upper disc member 4, 4′ and a lower disc member 6, 6′. (Useof a reference numeral with ′ thereafter refers to a second or furtherfeature equivalent to the feature indicated by the reference numeralalone. Thus, disc member 4 refers to the first prosthesis pair uppermember and disc member 4′ refers to the second prosthesis pair uppermember). The upper and lower disc members 4, 6; 4′, 6′ of each pairconstitute a left and right disc prosthesis respectively. These discmembers are shaped and dimensioned such that they can be inserted into alumbar disc space either side of the dural sac whilst taking intoaccount the posterior neural anatomy.

Each upper disc member 4, 4′ includes a vertebral endplate contactingsurface or superior surface 8, 8′ and an inferior surface 10, 10′. Eachlower disc member 6, 6′ includes a superior surface 12, 12′ and avertebral endplate contacting surface or inferior surface 14, 14′.

A dome shaped protrusion 16 is formed substantially centrally ofsuperior surface 12 of lower disc member 6. Protrusion 16 is received ina substantially central recess 18 on inferior surface 10 of upper discmember 4 as will be described in more detail below.

Both the superior surface 20 of upper member 4 and the inferior surface22 of lower member 6 are angled to provide the prosthesis with a “leadin” or narrowing tapered feature. This lead in feature allows theprosthesis to enter the posterior disc space which is narrower than theanterior disc space. In addition to the lead in feature which isprovided at the anterior or front end 24 of the prosthesis members, eachprosthesis pair has a lordosis or narrowing taper angle towards theposterior or rear end 26 of the prosthesis members between the inferiorsurface 14 of the lower disc member 6 and superior surface 8 of theupper member 4 of approximately 6 degrees (this angle or any other anglementioned hereinafter is for exemplary purposes and does not limit theinvention in any way), as shown in FIG. 1. This makes the outer surfacesof the disc prosthesis pairs at the posterior and anterior endssubstantially wedge shaped or tapered to allow ease of insertion intothe disc space. The taper or angle of the outer surfaces at theposterior end is in an opposite direction to the taper or angle of theouter surface at the anterior end.

With reference to FIGS. 1-27, the superior surface of upper disc member4 and the inferior surface of lower disc member 6 have attachment meansin the form of screws 28 to allow attachment of the disc prosthesis toadjacent vertebrae when positioned in a patient. Apertures 30 areprovided on the inferior and superior surfaces 10, 12 of the upper andlower disc members 4, 6 respectively of the prosthesis pairs to allowthe insertion of screws 28 through the prosthesis disc members. Thescrews are typically fitted from the posterior end 26 and the screws andinterior walls defining apertures 30 are provided at an acute angle tothe vertical to allow ease of attachment. Apertures 30 and screws 28within each pair diverge outwardly away from each other towards anteriorend 24. The inferior surface on which these apertures are provided onthe upper disc member is angled such that it slopes upwardly towards thesuperior surface of the upper disc member. The superior surface on whichthese apertures are provided on the lower disc member is angled suchthat it slopes downwardly towards the inferior surface of the lower discmember, thereby forming a narrowing taper at the posterior end of thelower disc member. These angled surfaces typically form, at least inpart, the posterior end segments of the inferior and superior surfaces48, 38 of the upper and lower disc members 4, 6 respectively. Thisfeature is to allow ease of insertion of screws or other attachmentmeans to anchor the prosthesis into the vertebra.

With the protrusion 16 on lower member 6 located in recess 18 of uppermember 4 when the prosthesis is assembled, relative movement between theupper and lower members 4, 6 allows the prosthesis to undergo extensionand flexion. The substantially dome shaped protrusion 16 contacts only apart of the recess 18 due to differences in symmetry and geometry. Inthe example illustrated, the upper member 4 can move with respect to thelower member 6 through approximately 10 degrees in a posterior directionto allow extension of the prosthesis, as shown in FIG. 2. Due to thenature of the inferior surface 10 of upper disc member 4, as the uppermember 4 goes into extension there is an increased distance adjacentanterior end 24 between the upper and lower members 4, 6. This resultsin a tightening of the annulus anteriorly, as is seen in physiologicalextension in the disc.

The upper member 4 can also move with respect to lower member 6 throughapproximately 10 degrees in an anterior direction to allow flexion ofthe prosthesis, as shown in FIG. 3. Again, due to the nature of inferiorsurface 10, as upper member goes into flexion there is in an increaseddistance adjacent posterior end 26 between the upper and lower members4, 6. This results in a tightening of the annulus posteriorly and servesas a natural block to further flexion.

Referring to FIGS. 13-21, there are shown more detailed views of lowerdisc prosthesis member 6, 6′. The dome shaped protrusion 16 is locatedin a central or intermediate section of superior surface 12 in theanterior to posterior plane. The anterior and posterior end sections 36,38 either side of the dome section 40, as shown by dotted lines 42, areprovided at an acute angle to the horizontal sloping downwardly fromintermediate section 40 towards ends 24, 26 respectively. The angledsurfaces 36, 38 are typically substantially planar in form. Thisdownwards incline is to accommodate the flexion and extension of theupper disc member 4 without impingement therewith. In the medial andlateral plane, protrusion 16 extends substantially the entire distancebetween the sides of the prosthesis member or from the medial to thelateral edges 32, 34 respectively.

Referring to FIGS. 22-27, there is illustrated more detailed views ofupper disc member 4 showing in particular the curvature and complexgeometry of the inferior surface 10. The surface 10 is divided intothree distinct regions as with the superior surface of lower disc member6; a substantially central or intermediate section 44 and anterior andposterior end sections 46, 48 respectively. Each section typicallyoccupies approximately one third of the inferior surface.

The central section 44 is substantially curved and forms an asymmetricalinverse dome shaped recess 18. The curvature of the recess in the medialhalf (i.e. portion adjacent medial edge 32) of the central sectiondiffers to the curvature of the recess in the lateral half (i.e. portionadjacent the lateral edge 34) of the central section. More specifically,the medial half of the dome has a curvature corresponding to the arc ofa circle having a radius of approximately 15 mm as shown by arrow 50,whereas the lateral half of the dome has a curvature corresponding tothe arc of a circle having a radius of approximately 40 mm, as shown byarrow 52 in FIG. 23. Thus, the curvature of the lateral half of thecentral section corresponds to an arc of a circle having a radius atleast double that of the arc of a circle corresponding to the curvatureof the medial half of the central section. The purpose of this asymmetryin the medial/lateral plane is to allow capture of the “dome” shapedprotrusion 16 of the lower disc member, but still allow some medial andlateral movement of the disc members. This contributes to stability andprevents dislocation of the upper and lower disc members duringmovement. The shorter curvature of the medial half of the centralsection relative to the lateral half helps in the medial and lateralmovement of the two vertebral bodies on the right and left prosthesis.

The anterior and posterior end sections 46, 48 are provided at an acuteangle to the horizontal, typically approximately 12-14 degrees as shownin FIG. 22, and provide an incline sloping downwardly from the centralsection 44 to the anterior and posterior ends 24, 26 of the inferiorsurface respectively. The surface of the anterior and posterior endsections that runs from the central section to the edge of the anteriorand posterior sections, as seen in FIG. 25, is generally planar,although a mild curvature or concavity to the surface can be provided.It is to be noted that the slope of the anterior section 46, 46′ endsbefore the anterior end, thereby leaving a small planar surface 54adjacent anterior end 46, 46′. In the lateral view, the edge of theanterior and posterior surfaces is substantially planar, and does notinclude a radius, so that the distance between the vertebrae increasesin flexion and extension.

The curvature of the central section recess on the anterior to posterioraxis corresponds to an arc of a circle having a radius of approximately17 mm, as shown by arrow 56 in FIG. 22. This is designed so that incombination with the other geometric features of the prosthesis,anterior and posterior movement of the upper disc member is allowed withrespect to the lower disc member of approximately 10 degrees, aspreviously described in relation to FIGS. 2 and 3, even if theprosthesis pairs are not exactly parallel to each other. This geometryis also designed to allow limited sideways (medial/lateral plane) tiltof the upper disc member on the lower disc member, allowing sidewaysmovement of the two disc members on each other, such that the right andthe left prosthesis move in tandem.

The geometry of the inferior surface of the upper disc member is alsodesigned to allow limited rotation of the right or left prosthesispairs. This means that if the upper disc member of the right prosthesispair moves anteriorly, the upper disc member on the left prosthesis pairmoves posteriorly allowing limited rotation of the two vertebral membersto occur.

The aim of the surgical procedure for the disc replacement is to insertthe left and right prosthesis pairs as parallel to each other aspossible within the disc space. However, even if there is medial tilt ofbetween 0-25 degrees between the prosthesis pairs, anterior andposterior movement of the pairs will still be possible and the upper andlower disc members will remain articulated during this movement. Thus,one disc pair is located at one side of the disc space and the otherdisc pair is located at the opposite side of the disc space (i.e in themedial lateral plane).

According to a further embodiment of the present invention, alternativeattachment means can be provided in the form of fin members 58 on thesuperior surface 8 of the upper disc member 4 and on the inferiorsurface 14 of the lower disc member 6, as shown in FIGS. 28-30 f. Finmembers 58 anchor the disc prosthesis to adjacent vertebrae.

Fins 58 are substantially triangular in shape and are providedlongitudinally of the prosthesis (i.e. between the posterior andanterior ends) towards the medial edge 32 of the disc members, so as toavoid the exiting nerve root which goes across the disc laterally. Morespecifically, the fins are located adjacent posterior end 26 and endbefore the lead in feature 20, 22 at the anterior end to avoid theexiting lumbar nerve root above the disc on the lateral side (i.e. theyextend for approximately two-thirds of the disc surface). The fin has anarrowing taper from posterior end 26 towards anterior end 24.

With the disc prosthesis pairs fitted, the upper disc members typicallymove substantially symmetrically on the lower disc members as the uppervertebral body moves forward on the lower vertebral body. The anteriorand posterior translations of these upper disc members are limited by atightening of the anterior and posterior annulus in flexion andextension. This is designed to reflect the physiological process bywhich the anterior and posterior annulus tightens in flexion andextension in a normal lumbar disc.

It is to be noted that the end parts of the lateral side 34 of the discmembers are curved to accommodate the lateral aspect of the disc spacewhich is similarly curved, thereby allowing better anatomical placement.Thus, the lateral side includes an intermediate substantiallystraight/linear/planar surface with the end portions either side thereofcurving inwardly towards the posterior and anterior ends respectively.The medial side 32 of the disc members is substantiallystraight/linear/planar.

Apertures 60 are defined in the posterior end 26 of the upper and lowerdisc members of each prosthesis pair to allow engagement of an insertiontool therewith so that the prosthesis pairs can be inserted into thedisc space. Apertures 60 are typically a spaced distance apart and theapertures on the upper disc member are substantially aligned with theapertures on the lower disc member.

According to a yet further embodiment of the present invention, a discprosthesis 502 is provided including two pairs of disc members, eachpair including an upper disc member 504, 504′ and a lower disc member506, 506′, as shown in FIGS. 59 a-59 e. The disc members can include anyor any combination of the abovementioned features and some of thesefeatures are labelled using the same reference labels as above. Inaddition, the vertebral endplate contact surface or superior surface508, 508′ of disc members 504, 504′ are provided with a convex shapedportion 510, 510′ thereon. The shape of the convex shaped portion issubstantially complementary to the recess or concave shaped portion toprovide an improved fit when the disc prosthesis is located with thevertebral body of an adjacent disc in use.

The height of the convex portion 510, 510′ is greater towards the medialside 512 than to the lateral side 514. The longitudinal axis or longestlength of the convex portion is along the anterior 516-posterior 518axis. In addition, the convex portion is closer to the posterior end 518of the disc than to the anterior end of the disc.

Facet Joint Replacement Prosthesis

Referring to FIGS. 31-52 b, there is illustrated facet joint replacementprostheses 102, 202 according to embodiments of the present invention.Facet joint prostheses 102, 202 can be used alone, or in combinationwith the lumbar disc prosthesis described above to form a single unitsystem is the prostheses are designed to replace the entire facet jointon both the right and the left side of the vertebrae.

Prosthesis 102 includes a first member or male member 104 and a secondmember or female member 106. Both male and female members 104, 106 aresubstantially elongate in form. Male member 104 has a first end 108 withsecuring means in the form of an aperture 110 defined therein and asecond end 112. End 108 is in the form of a flat or planar plate portion113 and a curved arm portion 114 is provided between this plate portion113 and end 112. Female member 106 has a first end 116 with securingmeans in the form of an aperture 118 defined therein and a second end120. End 116 is in the form of a flat or plate portion 117 and a curvedchannel portion 122 is provided between this plate portion 117 and end120.

The facet prosthesis 102 is inserted by removing the entire existingfacet joint and placing pedicle screws 124 into the vertebral body aboveand below the disc. End 112 of male member 104 is inserted into end 120of channel 122 of female member 106 and the ends of the pedicle screws123 are threaded through apertures 118 and 110 and secured with a nut126. The edge of plate portions 113 and 117 which define apertures 118and 110 are tapered inwardly so as to allow a good fit with a tapered orconical shaped locking nut 126, as shown in FIG. 50. A further spacingor locking nut 127 can also be provided.

Male member 104 has a rear or posterior 128 and a front or anteriorsurface 130. The anterior surface 130 of arm portion 114 is typicallysubstantially concave in shape and the posterior surface 128 of armportion 114 is typically substantially convex in shape. As such, whenmember 104 is fitted to pedicle screw 124 in use, anterior surface 130typically faces the vertebrae to which it is to be attached.

Female member 106 has a rear or posterior surface 132 and a front oranterior surface 134. The anterior surface 134 of channel portion 122 istypically substantially concave in shape and the posterior surface ofchannel portion 122 is typically substantially convex in shape. As such,when member 106 is fitted to pedicel screw 124 in use, anterior surface134 typically faces the vertebrae to which it is to be attached.

Thus, with the female member interconnected to the male member, theprosthesis curves outwardly from the vertebrae and outwardly of theplate portions 117 and 113. Arm portion 114 of male member 104 is freelyslidable or movable in channel portion 122 of female member 106. Thecurvature of the male and female members is substantially the same. Themale member 104 is of slightly smaller dimensions than the interiordimensions of channel portion 122, such that there is a small gaptherebetween to allow some sideways movement, as shown by arrows 150 inFIG. 47, and rotation of the male member in the female member, as shownby arrows 152 in FIG. 46. The male member can also move longitudinallyin the female member to allow flexion and extension of the prosthesisand thus the vertebral body, as shown by arrows 154 in FIGS. 45 a and 45b.

End 120 of channel portion 122 has curved ends 136 and straight sidewalls 138 or side walls of less curvature than said curved ends to forma flattened oval shape, as shown in FIG. 49. Preferably side walls 138form part of an arc, the radius of which is larger than for the arc ofthe curved ends 136. End 112 of arm portion 114 is substantiallycomplementary in shape to end 120 having curved ends 140 and straightsidewalls 142 or side walls of less curvature than said curved ends.Preferably side walls 142 form part of an arc, the radius of which islarger than for the arc of the curved ends 140. The provision of theshaped ends allows a small degree of rotation of the male component inthe female component.

The anterior surface 134 of channel portion 122 has a slot 144 providedlongitudinally thereof. Slot 144 is of such a width that the channelportion encloses approximately 70% of the male member 104 wheninterconnected therewith and is provided to allow some lateral flexionand/or extension and rotation between the male and female members.

A left and right pair of facet joint prostheses are located between thedisc on each side thereof. It is irrelevant whether the female or themale member is uppermost and thus the position of the members isinterchangeable. In addition, the facet prosthesis can be used at twoadjacent levels, and anchor into pedicle screws, as seen in FIGS. 48a-48 d. A middle pedicle screw 124′ can be used to accommodate two plateportions of the male and/or female members to form a stack. Any numberof members can be attached to a pedicle screw as required.

The prosthesis can be formed from any suitable material, such as forexample, stainless steel, ceramics, titanium, carbide or other suitablemetal alloys. The surface of the prosthesis can be provided slightlyroughened so as to increase bonding of the same with bone and/or one ormore surface coatings can be provided thereon, such as for example,hyroxyapitite or plasma spray.

Referring to FIGS. 51-52 b, prosthesis 202 is similar in appearance toprosthesis 102 and the same components are referred to in the twoprostheses using the same reference numerals. However, in thisembodiment, sliding or telescopic movement of male member 104 in channelportion 122 of female member 106 is limited firstly by locking means inthe form of a locking screw 204, and secondly by guide means in the formof enclosed slot 206 and pin 207.

The portion 208 adjacent end 112 of male member 104 is of smallerdimensions than the remaining portion 210 of male member 104 adjacentend 108. A shoulder portion 212 is created between portions 208, 210and, with male member 104 in a fully retracted position as shown in FIG.52 a, end 120 of female member 106 can be located in abuttingrelationship with shoulder portion 212 or a small spaced distance apart.

Portion 208 of male member 104 is provided with a plurality of recesses214 at spaced apart locations along an anterior surface thereof. An endof locking screw 204 located through an aperture provided on theposterior surface of female member 106 is typically located in one ofthese recesses 214 if the position of the male member with respect tothe female member is required to be locked.

Telescopic or sliding movement between female member 106 and male member104 can be limited via enclosed slot 206 provided on an posteriorsurface of female member 106 adjacent end 120. A pin 207 provided on ananterior surface of male member 104 adjacent end 112 thereof is slidablymounted in slot 206, thereby guiding movement of the male member withrespect to the female member and preventing complete dissociation of thetwo members from each other.

A further difference between prosthesis 202 and prosthesis 102 is thatends 108 and 116 of male member 104 and female member 106 aresubstantially rounded and continuous in form. These ends can be attachedto a clamp and pedicle screw to allow attachment of the same to therequired vertebral bodies above and below the disc.

Referring to FIGS. 60 a-61, there is illustrated a further embodiment ofa facet joint prosthesis 600 according to the present invention. Theprosthesis includes a male member 602 and a female member 604 which aresimilar in form to that described above and similar reference numeralsare used to described corresponding parts. The difference of prosthesis600 compared to the previously described embodiments are that themovement of male member 602 relative to female member 604 is limited bytwo clamping rings 606, 608. Thus, the clamping rings provide a clampingor compressive force around the two members when joined together tolimit movement of the members relative to each other.

Clamping rings 606, 608 each have a sleeve portion 610 which is locatedaround member 604, adjacent open ended slot 122 defined in said member604. Plate members 612 protrude outwardly from the aperture 614 definedwithin sleeve portion 610 to define free ends of said clamping ring. Theplate members 612 are a spaced distance apart and substantially parallelto each other. An aperture defined in each plate member allows aclamping screw 616 to be located therethrough so that a clamping forcecan be exerted via the clamping ring by tightening or loosening thescrew.

A projection 618 is provided on an interior surface of sleeve portion610 and said projection 618 is located in slot 122 in use to restrictmovement of the clamping ring to sliding movement along slot 122.

The distance between clamping rings 606, 608 when located around femalemember determine the range of movement of male member 602 relative tofemale member 604 by limiting movement of pin 207 on male member 602.Thus, the clamping rings allow the facet joint prosthesis to undergomicromotion when the rings are close together or full/greater movementwhen the clamping rings are a greater distance apart from each other.This flexibility in movement ensures greater versatility of theprosthesis and allows the same to be multi-functional.

A stop member 620 can also be inserted in the channel 622 defined infemale member 602 to limit movement of male member 602 in said channel.The free end of male member 602 is substantially rounded.

Thus, it can be seen that in one aspect of the present invention, thetwo pairs of lumbar disc prosthesis and the two pairs of facet jointprosthesis can be used to form a system designed to allow anarthroplasty to be performed through the posterior route, allowingmovement between the vertebral bodies as well as restoring stabilitybetween the two vertebral bodies by allowing normal load transmissionacross the disc, freeing up the neural structures and replacing thefacet joints as well.

Lumbar and Cervical Disc Prostheses for Insertion Via an Anterior Route

Referring to FIGS. 53 a-58 e, there is illustrated lumbar and cervicaldisc prostheses 302, 402 respectively for insertion via an anteriorroute as a replacement for a diseased and/or damaged lumbar or cervicaldisc.

The disc prostheses 302, 402 include a pair of disc member having anupper disc member 304, 404 and a lower disc member 306, 406. The upperdisc members include a vertebral endplate contacting surface or superiorsurface 308, 408 and an inferior surface 310, 410. The lower discmembers include a superior surface 312, 412 and a vertebral endplatecontacting surface or inferior surface 314, 414.

The disc prostheses for insertion via an anterior route are the same asthe lumbar disc prostheses for insertion via a posterior route apartfrom the following differences.

The protrusion 316, 416 provided on the superior surfaces 312, 412 ofthe lower disc members 306, 406 is rugby ball shaped rather than domedshaped. Thus, the radius of the curve of the protrusion in the mediallateral plane 318, 418 is larger than the radius of the curve in theanterior posterior plane 320, 420.

In the cervical prosthesis, the protrusion 416 extends for substantiallythe entire width in the medial-lateral plane and the anterior-posteriorplane of the disc prosthesis.

The inferior surface 310, 410 of the upper disc members 304, 404 isdivided into three sections a middle section 322, 422 and two endsections 324, 424 and 326, 426. The boundaries of the sections areprovided in the medial-lateral plane 318, 418. The superior surface 312of the lower disc member 306 can also be divided into similar threesections in the lumbar disc prosthesis.

The recess portion 328, 428 is provided in the middle section 322, 422as with the earlier embodiments. The recess portion 328, 428 issubstantially curved and the curvature of the medial part 330, 430 ofthe recessed portion is substantially symmetrical to the curvature ofthe lateral part 332, 432 (i.e. symmetrical about the midline). Thus,the lateral part 332, 432 has curvature corresponding to an arc of acircle which has a radius substantially equal to that of an arc of acircle corresponding to the curvature of the medial part 330, 430.

The radius of the curve of the recess portion 328, 428 is substantiallyequal to the radius of the curve of the protrusion 316, 416 in themedial-lateral plane 318.

Attachment means in the form of screws 334, 434 are provided adjacent ananterior edge 336, 436 of both the upper and lower disc prostheses. Onescrew is provided in the cervical prosthesis and two screws are providedin the lumbar prosthesis. Further attachment means in the form of spikes338, 438 can be provided on the vertebral endplate contact surfaces toallow increased engagement with an adjacent vertebral disc member inuse.

Lordosis is built into the prosthesis so that the anterior edge 336, 436is larger in height than the posterior edge 338, 438.

What is claimed is:
 1. A lumbar disc prosthesis defining amedial-lateral plane and a anterior-posterior plane, the lumbar disccomprising: a first pair of disc members including a first member havinga vertebral endplate contact surface and a recessed portion on anopposing surface thereof, the recessed portion defining a curvature inthe medial-lateral plane defining a plurality of different radii values,the recessed portion extending along substantially the entire width ofthe medial-lateral plane and a second member having a vertebral endplatecontact surface and a protruding portion on an opposing surface thereof,the protruding portion of the second member engaging the recessedportion of the first member during use; and a second pair of discmembers, said second pair of disc members also including first andsecond members.
 2. The prosthesis of claim 1, wherein the recessedportion defines a curvature in an anterior-posterior plane that isdifferent than the curvature in the medial-lateral plane.
 3. Theprosthesis of claim 1, wherein the recessed portion defines a curvaturein an anterior-posterior plane that is symmetrical about a midpoint ofthe curvature.
 4. The prosthesis of claim 1, wherein the curvature inthe medial-lateral plane is asymmetrical about a midpoint of thecurvature.
 5. The prosthesis of claim 4, wherein the curvature in themedial-lateral plane defines a medial radius less than a lateral radiusdefined by the curvature.
 6. The prosthesis of claim 1, wherein theprotruding portion defines a curvature in an anterior-posterior planethat is different than a curvature of the protruding portion defined ina medial-lateral plane.
 7. The prosthesis of claim 1, wherein theprotruding portion defines a curvature in an anterior-posterior planethat is the same as a curvature of the protruding portion defined in amedial-lateral plane.